Power presses in which one platen moves towards another operate under tremendous forces. Unfortunately, such presses also generally require their human operators to, at some point in the operating cycle, place their hands between these platens. Unfortunately, on occasion the expectable results occur; workers with imperfect coordination fail to timely withdraw their hands from between the platens during the operation of the press. Many workers have suffered devastating injuries in this fashion.
To avoid the recurrence of such tragedies, various types of safety devices have found their way into this type of press. Some of these devices take the form of switches requiring a timed relationship in order to operate the press properly. These switches do not allow the movable platen to approach the other unless and until the operator has closed both switches at a substantially simultaneous time. Moreover, the operator cannot tape the switch closed since they must both open and close to operate. This type of device has the effect of requiring the operator to move his hands out of the working area before he can effectuate the operation of the press. Due to these switches, the accident record for such presses has drastically improved.
However, these coordinated hand switches, although beneficial, have not produced entirely accident-free operations. The controls of the machine may, for example, degenerate through long wear and allow the platens to close with their usual force. Since this may occur without the receipt of the required signals from the hand switches, the platens may close with the worker's hands in the path of the press. Thus, serious injury can still occur.
Furthermore, the movable platen on a press often occupies a position over the stationary platen. Opening the press requires moving the platen to an elevated position. There the press's mechanism holds the platen to provide the work area for the operator.
However, the mechanism supporting the movable platen in the elevated position may also degenerate through wear. Accordingly, although the controls indicate that the upper platen should remain elevated, it may slip down towards the fixed platen. Although not moving under full force of the press, it can still contact the operator's hands and injure him.
Accordingly, press manufacturers have sought further safety devices to guard against these albeit infrequent mishaps. Generally, they employ some sort of physical obstruction that prevents the moable platen from moving towards the other. This obstruction will generally have sufficient strength to withstand the full force of the press. Additionally, the manufacturers have tried to automate the placement and removal of the obstruction in the path of the platen. To do so, they have coordinated its motion with the happening of a separate event. This automation seeks to eliminate reliance upon the operator's memory to use this accident-preventing mechanism. It also eliminates the incorporation of an additional step in preparing the press for a cycle of operation.
T. F. Stacy, in his U.S. Pat. No. 2,240,630, shows a device which prevents the upper platen, when raised, from slipping and closing under the action of gravity and injuring the operator. To provide this protection, Stacy moves a pin into and out of the path of a bracket attached to the upper platen. However, this device provides absolutely no protection against the unintended closure of the platens under full force. Consequently, a mishap in the controls producing such a forced closing of the platens would still effect tragic consequences upon the operator.
In U.S. Pat. No. 2,946,277, L. F. Archer provides a rocker-arm device which rotates towards and away from the path of a press's moving platen. Neither of these positions fully removes the rocker arm from the platen's path. More significantly, when the rocker arm engages the moving platen, the full force propelling the platen becomes applied directly to the shaft on which the rocker arm pivots. After a number of these engagements, the shaft may very likely become disfigured or ruined due to the tremendous stress it withstands. Furthermore, the shaft could break in use, resulting in the disappearance of the intended safety measure.
Alternatively, making Archer's pivotal connection sufficiently strong to withstand such abuse would require a massive shaft with relatively rigid connections. This massive construction would impede the movement of the rocker arm between its positions, require a strong force to achieve these positional changes, and increase the expense of the overall unit.
Moreover, Archer simply operates the rocker arm off a direct mechanical link to one of the two hand-actuated valves applying the power to the press. Inadvertent actuation of this valve can result in the safety moving out of its protecting position at an undesired time.
F. M. Adair et al., in their U.S. Pat. No. 2,990,578, show two bell cranks which move between the platens when a guard covering the press's front opens. The pivot points of the bell cranks remain in front of the press to allow mechanical actuation by rollers attached to the guard. However, with the lever arms of the bell cranks lying in front of the press, they become vulnerable to accidental actuation and thus removed from the protecting position. Additionally, their location in front of the press interferes with its ordinary operation.
Additionally, a piece of foreign material can accidentally lodge between the bell cranks' bars and the lower platen. This material could derive from the actual work pieces in the pess itself. This material, however, could completely prevent the bars from assuming their protecting position and result in an unsafe machine.
Moreover, the roller mechanism on the guard could prevent the cranks from rotating completely into their position between the platens. Were that to happen, the pivot point of the bell cranks could absorb substantial amounts of the force propelling the platens together.
E. L. Cornell, Jr., in his U.S. Pat. No. 3,143,957, and E. W. Anderson, in his U.S. Pat. No. 3,541,950 show protective devices which rotate into the path of the moving platen. Cornell's device incorporates a rocker arm which pivots out of the platen's path when a feeler moves sufficiently downward to indicate the absence of a hand in the work area. Anderson partially removes his bar when the operator's hand actuates the operating switch to which the bar connects. However, should the rocker arm in either patent actually contact the moving platen, the pivotal connections of the devices receive the full force propelling the platen. As a result, these pivotal connections can suffer severe damage and totally lose their usefulness.
J. Scutella's U.S. Pat. No. 3,554,067 shows a safety guard operated on pneumatic and electric circuits. When a guard passes in front of the work area, which it presumably can do only upon the absence of an appendage in that area, the circuits allow the hydraulic press to operate. No mechanism appears, however, to prevent the descent of the platen at an unpropitious moment. Accordingly, even though the electric and pneumatic circuits operate properly, a malfunction in the hydraulic circuit may cause the press to descend.
While the devices of present presses have effected significant improvements in their safety records, accidents still occur. These unfortunate occasions illustrate the need for additional innovations that can further prevent or at least minimize the opportunity for injuries to workers.